This invention relates generally to apparatus and methods for producing aqueous chlorine dioxide solutions, and particularly to such apparatus and methods that utilize dry chemicals that react to form chlorine dioxide when exposed to water.
Chlorine dioxide is an excellent disinfectant and oxidizer with bleaching, deodorizing, bactericidal, viricidal, algicidal and fungicidal properties. It is frequently used to control microorganisms on or around foods because it destroys the microorganisms without forming byproducts that pose a significant adverse risk to human health, e.g., chloramines and chlorinated organic compounds. Chlorine dioxide is an effective antimicrobial agent at a concentration as low as 0.1 ppm and over a wide pH range. It is thought to penetrate cell walls and cell membranes and react with vital amino acids in the cytoplasm of the cell to kill the organism.
Unfortunately, chlorine dioxide is not stable during storage and can be explosive at high concentrations. As a result, chlorine dioxide gas is not produced and shipped under pressure. It must generally be generated on site using conventional chlorine dioxide generators or other means of generation. Conventional chlorine dioxide generation can be carried out in an efficient manner in connection with large-scale operations such as those in pulp and paper or water treatment facilities. In other applications, however, generating chlorine dioxide on site is not a good option. Conventional on-site chlorine dioxide generation can be costly, cumbersome and difficult because of the need for a generator and the need to handle the generator and the chemicals associated with the generation process.
Chlorine dioxide can also be generated by combining chlorite anions and acid in an aqueous solution. Typically, an acid is added to a solution containing in the range of from about 0.01 to about 32 percent by weight sodium chlorite and having a pH in the range of from about 8 to about 13. The acid can be any acid capable of lowering the pH of the solution to a level below about 7. For example, when approximately 10 grams of citric acid powder are added to an aqueous solution containing approximately 3.35% by weight sodium chlorite, the pH of the solution is lowered to about 2.9 and a solution containing approximately 7% by weight chlorine dioxide is formed.
A solution of a metal chlorite and water wherein the pH of the solution is maintained at 8 or above is sometimes referred to as a stabilized chlorine dioxide solution. Unfortunately, stabilize chlorine dioxide solutions are of limited use if they are needed at remote locations because of the difficulty and expense associated with handling and shipping the solutions. Also, in order to activate a xe2x80x9cstabilizedxe2x80x9d chlorine dioxide solution, the pH of the solution must be lowered to below 5, typically to a range of from about 2 to about 3. Although lowering the pH of the solution to such a level can be done on site, it is not typically a good alternative because of the danger associated with handling acids manually (e.g., the danger associated with inadvertent skin contact and inhalation of acid vapors).
In order to avoid the difficulty of using conventional chlorine dioxide generators, the expense associated with handling and shipping stabilized chlorine dioxide solutions and related precursor solutions and the dangers associated with activating chlorine dioxide solutions, dry compositions containing chemicals (e.g., sodium chlorite and acid) that react to form chlorine dioxide when placed in water have been developed. The compositions can be easily shipped to remote locations in dry form. The necessary water can be merely added on site. For example, in an application wherein a disinfectant solution is needed to clean surfaces, a dry composition containing a metal chlorite and an acid can be mixed with water on site which causes the components to react and produce an aqueous chlorine dioxide solution. The solution is then used to disinfect the surfaces. The aqueous chlorine dioxide solution is produced (chlorite anion is converted to chlorine dioxide) according to the following equation:
5ClO2xe2x88x92+5H+xe2x86x924ClO2+HCl+2H2O
Dry compositions for generating chlorine dioxide solutions are known in the art. For example, U.S. Pat. No. 2,022,262, issued to White on Nov. 26, 1935, discloses stable stain-removing compositions made from a dry mixture of water-soluble alkaline chlorite salt, an oxalate and an acid. U.S. Pat. No. 2,071,091, issued to Taylor on Feb. 16, 1937, discloses the use of chlorous acid and chlorites to kill fungi and bacterial organisms by exposing the organisms to the compounds at a pH of less than about 7. The patent also discloses using dry mixtures of chlorites and acids to produce stable aqueous solutions useful as bleaching agents. U.S. Pat. No. 2,482,891, issued to Aston on Sep. 27, 1949, discloses stable, solid, substantially anhydrous compositions comprising alkaline chlorite salts and organic acid anhydrides which release chlorine dioxide when contacted with water.
Canadian Patent No. 959,238, issued to Callerame on Dec. 17, 1974, discloses using two water-soluble envelopes, one containing sodium chlorite and one containing an acid, to generate chlorine dioxide. The envelopes are placed in water and the sodium chlorite and acid dissolve in the water and react to produce a chlorine dioxide solution. U.S. Pat. No. 2,071,094, issued to Vincent on Feb. 16, 1937, discloses deodorizing compositions in the form of dry briquettes formed of a mixture of soluble chlorite, an acidifying agent, and a filler of relatively low solubility. Chlorine dioxide is generated when the briquettes contact water.
U.S. Pat. No. 4,585,482, issued to Tice et al. on Apr. 29, 1986, discloses a long-acting biocidal composition comprising a microencapsulated mixture of chlorite and acid that when added to water releases chlorine dioxide. The primary purpose of the microencapsulation is to provide for hard particles that will be free flowing when handled. The microencapsulated composition also protects against water loss from the interior of the microcapsule. The microcapsules produce chlorine dioxide when immersed in water. Unfortunately, the microcapsules release chlorine dioxide relatively slowly and are therefore not suitable for applications that require the preparation of chlorine dioxide on a relatively fast basis.
PCT Application PCT/US98/22564 (WO 99/24356), published on May 20, 1999, discloses a method and device for producing chlorine dioxide solutions wherein sodium chlorite and an acid are mixed and enclosed in a semi-permeable membrane device. When the device is placed in water, water penetrates the membrane. The acid and sodium chlorite dissolve in the water and react to produce chlorine dioxide. The chlorine dioxide exits the device through the membrane into the water in which the device is immersed producing a chlorine dioxide solution that can be used as an anti-microbial solution or for other purposes. The primary disadvantage of the disclosed device and method is that ambient moisture can penetrate the semi-permeable membrane and initiate the reaction prematurely.
In general, the prior art devices and methods using membranes are susceptible to premature activation by water or water vapor and therefore have a reduced shelf life unless sufficient steps are taken to protect the devices from exposure to ambient moisture or water. Such devices and methods are typically slow to interact with water and produce the desired chlorine dioxide. Also, in order to comply with U.S. Department of Transportation and other regulations, many prior art devices require that special and sometimes burdensome handling and shipping procedures be utilized in connection with the devices. For example, if sodium chlorite and acid are packaged together, certain restrictions may apply.
As a result, there is a need for a device for producing an aqueous chlorine dioxide solution that has an extended shelf life compared to prior art devices, that is not susceptible to activation by ambient moisture, that forms a chlorine dioxide solution much more quickly than prior art devices and that can be assembled and packaged in ways that avoid stringent handling and shipping regulations.
In accordance with the invention, a device for producing an aqueous chlorine dioxide solution when exposed to water is provided. The device comprises a membrane shell defining a compartment which includes one or more dry chemical components capable of producing chlorine dioxide gas when exposed to water. Wick means are connected to the membrane shell and extend into the compartment for absorbing water and transporting water into the compartment whereby when the device is exposed to water the wick member absorbs water and transports water into the compartment, the chemical component(s) dissolve in the water and produce chlorine dioxide gas in the compartment, and chlorine dioxide gas exits the compartment through the membrane shell.
In a preferred embodiment, the compartment of the device includes a metal chlorite component and an acid component. In use, for example, the device is submersed in a container of water. The wick means quickly absorbs water and transports the water into the compartment. Metal chlorite and acid in the compartment then dissolve in the water and react to produce chlorine dioxide gas in the compartment. The chlorine dioxide gas passes through the membrane shell and transforms the water in the container into a chlorine dioxide solution. The solution can be used, for example, to disinfect surfaces or for a variety of other purposes as known in the art.
In one embodiment, the membrane shell is substantially impervious to liquid (e.g., water) but permeable to gas (e.g., chlorine dioxide gas). In another embodiment, the membrane shell is permeable to both liquid (e.g., water) and gas (e.g., chlorine dioxide gas).
In another embodiment, the wick means is a wick member having a first end that extends into the compartment and an opposing second end that extends beyond the outer edge of the membrane shell. In yet another embodiment, the wick means is a wick member that divides the compartment into first and second compartment sections. For example, the first compartment section can contain exclusively the metal chlorite component and the second compartment section can contain exclusively the acid component. This helps prevent the metal chlorite component and acid component from prematurely reacting. For example, in the event that a small amount of moisture accumulates in the device, the wick member will prevent a reaction that might otherwise occur. In another embodiment, the wick means is a wick member that divides the compartment into a plurality of compartment sections. For example, the metal chlorite component can be isolated in one compartment section, the acid component can be isolated in a second compartment section, and a surfactant or some other additive can be isolated in a third compartment section. In another embodiment, the wick means comprises at least two wick members, the wick members dividing the compartment into at least two compartment sections.
The inventive device is very useful for relatively small applications, for example where expensive chlorine dioxide generation equipment is not economically feasible.
An important advantage of the inventive device is that it can be modified to meet applicable shipping and handling regulations. For example, in one embodiment, the device is packaged to include a metal chlorite component together with any additives employed for the particular application. The compartment of the device includes a sealable opening therein for allowing the acid component to be placed in the compartment at the point of use (e.g., just prior to submersing the device to water). The acid component is separately packaged with the device; for example, it can be in tablet form. Prior to placing the device in water, the user merely inserts the acid in the device and seals the opening. By packaging the acid component separately, inadvertent premature exposure of the device to water (even a substantial amount of water) will not cause the chemicals to react. As a result, less stringent regulations regarding shipping and handling the device may apply.
If necessary or desirable for the end-use application, a weight can be attached to one of the membrane shell and the wick means (e.g., placed into the compartment) or otherwise incorporated into the device (e.g., formed as part of the wick member) to ensure that the device is immersed when it is placed in a body of water. Although it is not critical to do so, the device can also be packaged in a water-resistant envelope (e.g., a foil pouch) in order to minimize the risk of the device being inadvertently exposed to water prior to use. Also, for particular applications, the membrane shell can include a plurality of small openings for facilitating the passage of liquid (e.g., water) into and out of the device and decreasing the time required for the chlorine dioxide solution to be produced.
It is, therefore, an object of the present invention to provide a device that effectively produces an aqueous chlorine dioxide solution when exposed to water but has a stable shelf-life prior to exposure to water.
It is also an object of the present invention to provide such a device that is not susceptable to activation by ambient moisture.
It is a further object of the present invention to provide such a device that is less costly to produce and manufacture than other devices for producing aqueous chlorine dioxide solutions presently on the market.
It is yet another object of the present invention to provide a device that produces an aqueous chlorine dioxide solution in a relatively short amount of time when compared to prior art devices.
Still another object of the invention is to provide such a device that can be modified to meet applicable shipping and handling regulations.